I'm not a regular user of 4chan, but I am familiar with how it and other imageboards work. Basically you have a collection of threads where users can post images, text, and short videos. No accounts are required and everything is anonymous (well not really, more on that later). Perhaps one of the most distinguishing things about imageboards compared with traditional forums is that the threads aren't permanent. A typical thread on 4chan has a lifetime measured in minutes. When a new thread is created, the thread which hasn't been replied to for the longest amount of time drops off the site's catalog and is deleted forever.

This automatic pruning of threads also makes it a prime application for the blockchain. The advantages of having a blockchain include higher resilience and redundancy as it is impervious to hacks, no censorship, superior privacy, and true anonymity. 4chan isn't really anonymous because records of your IP and machine details are stored in centralized servers and when faced with legal action, it would be trivial for governments to gather this information and use it to find out your real identity. The Sarah Palin email hacking incident is an example of where such a thing did actually occur.

The technical problems facing a decentralized imageboard are similar to that of Bitcoin. Resistance to Sybil attacks could be enforced by adoption of a traditional proof-of-work scheme or a proof-of-stake scheme similar to Peercoin. An internal currency of credits could insure that there is some cost to creating a thread or at least insure that the network cannot be attacked by flooding the catalog with thousands of spam or useless threads. Nodes could be rewarded with credits based on their ability to sustain the network, etc. Alternatively, an existing cryptocurrency such as Bitcoin or Litecoin could also be co-opted for this use.

This is all just an idea and unfortunately, my programming skills are nowhere near as good enough to create something as complex as this. But someone who is skilled at programming and is interested in creating a more open, decentralized Internet where humans can interact and share ideas freely without the threat of persecution by oppressive regimes could create this.

So a blockchain jam-packed with child porn, pictures of chicks with dicks, and forever alone rants. I'll pass.

Just because it happens to be a chan-style imageboard doesn't necessarily mean that it has to adopt chan culture. It could be a platform for people living in oppressive regimes to discuss their thoughts and opinions as well as a platform for organizations such as WikiLeaks. It could be transformed into a microblog or an image gallery or a forum too. The purpose would be similar to Tor but sites on Tor rely on centralized servers which can be shut down.

There would be separate boards for different topics so it would make little sense for someone to post a forever alone rant or child porn or pre-op transsexuals on a board about politics or photography or religion for example since their thread would just get ignored and buried.